The present invention generally relates to an annealing process and more particularly, to a continuous annealing furnace for annealing a strip at relatively high speed, with the strip being kept superior in tracking properties, even though it is thin.
Conventionally, a continuous annealing furnace for a steel strip has been generally classified into a horizontal type or vertical type. Either one of totally horizontal type, multilayer type, L-shaped type, inclined type, combination type of horizontal and vertical types, and totally vertical type is selected as a furnace type according to production capacity of a material to be treated, kind thereof, object of treatment therefor or the like.
Furthermore, in a transporting means for the strip in the continuous annealing furnace, there exist transport systems of roll type, catenary type, floating type or the like.
Meanwhile, when the strip is produced through the continuous annealing treatment at a throughput of 20,000 to 40,000 tons a month, as disclosed in Japanese Patent Publication Tokkosho No. 59-42732, a totally vertical type furnace wherein each of a heating zone, a soaking zone, a first cooling zone (an overaging treating zone) and a last treating zone is vertically arranged, is generally employed in achieving the above described capacity.
Moreover, a combination of horizontal type furnace and vertical type furnace is occasionally employed for the continuous annealing furnace, as disclosed in Japanese Patent Publication Tokkosho No. 56-18053 wherein a direct firing type heating zone is horizontally arranged and other zones are vertically arranged, or as disclosed in Japanese Patent Publication Tokkosho No. 58-53052 wherein, after quenching treatment of the material, the material is led into an overaging treating zone which is a horizontal type furnace having horizontal multi-row passage for the material. In each of the continuous annealing furnaces as described in the aforementioned disclosures, the strip is passed within the furnace so as to be subjected to the annealing treatment, while it is supported by rolls.
In addition, there also exists another combination type furnace wherein the heating zone and the soaking zone are arranged in the horizontal type furnace and the cooling zone is arranged in the vertical type furnace and furthermore, bridle roll means are provided at an inlet side of the horizontal type furnace and at an outlet side of the vertical type furnace so as to control the strip in tension.
However, in this kind of the continuous annealing furnace, or in the totally vertical type furnace which is effectively employed for continuous annealing at a monthly production of 20,000 to 40,000 tons per unit as stated hereinbefore, and in decreasing the installation space thereof, it is extremely difficult to conduct heat treatment of for example, a thin strip having a thickness of 0.07 mm and a width of 1,270 mm at such a relatively high speed as 600 m/min.
That is to say, although the strip is necessarily applied with a tension of, for example, 0.5 to 1.0 kg/mm.sup.2 in order not only to be steadily transported at the aforementioned speed without meandering thereof, but also to keep the superior tracking properties thereof, such tension results in that a constriction (a heat buckle or a tension buckle) is undesirably produced on the strip in a portion of the annealing furnace which is high in temperature.
Furthermore, since the strip is controlled to be constant in tension as a whole by the bridle roll means arranged at both inlet and outlet sides of the annealing furnace as described hereinbefore, the strip is naturally limited in size. In dimensions, the strip is limited to be approximately 0.15 mm in thickness and approximately 920 mm in width and as a result, it is practically impossible to conduct the continuous annealing treatment on the strip having a thickness smaller than the above described value or that having a width larger than the above described value.
Moreover, since a plurality of rolls are employed for the transporting means, there has been a drawback that in order to prevent the occurrence of construction of the strip, such a countermeasure is required as to eliminate temperature difference between the material and the rolls arranged in the heating zone and the soaking zone.
On the other hand, in the combination type continuous annealing furnace of the horizontal and the vertical types, although the occurrence of constriction can be decreased to some extent, as compared with a totally vertical type furnace owing to the fact that the tension on the strip in the horizontal type furnace is relatively smaller than that in the vertical type furnace, there has been such a problem that not only a large number of transporting rolls are necessarily required, but also the strip is liable to be undesirably damaged by the rolls.
The present invention has been developed in view of the facts that the occurrence of constriction of the material is caused by the tension applied thereon and the heating conditions therefor in the heating zone and the soaking zone, and the material is limited in size by the amount of the tension exerted thereon which is required for steady transportation thereof in the aforementioned zones, and the heat buckle or the tension buckle tends to occur in the heating and the soaking treatments of the material.
Furthermore, the present invention has also been developed on the basis of the facts that the strip in the horizontal type furnace can be decreased in tension relatively smaller than that in the vertical type furnace, and the continuous annealing furnace can be decreased in installation space thereof by arranging a portion of the material which is relatively low in temperature, in the vertical type furnace, and the tension of the material in each furnace of the combination type can be independently controlled by arranging additional bridle roll means between both furnaces.